Generally, display devices are classified into two types; passive type displays and active type displays. The passive type display generally comprises a light source and a display panel. Light from the light source passes through the display panel and is modulated in transparency to form images or characters. As a result, images or characters are displayed on the display panel. In the passive type display, there is a liquid crystal display, an electrochemical display and an electrophoretic display. A liquid crystal display is typically used for a flat display.
The active type display has a vacuum-tube or a tube in which inert gas is sealingly contained. The vacuum-tube or other sealed tube has a light emitting device of its own. Such tube also has a display screen. The light emitting device emits light, for example, by electric discharge or phosphor excitation by electrons. Images and characters are displayed on the display screen by controlling the light emission of the light emitting device. In the active type display, there is a cathode ray tube (CRT), a gas-electric discharge device, and a display panel comprising light emitting diodes arranged in a matrix form. For a flat display, a plasma display panel (PDP), which is a gas electric discharge device, is typically used.
In the PDP, under relatively high gas pressure, cold cathode emission is achieved between two electrodes having a narrow gap therebetween and, as a result, glow discharge light emission is achieved. In the glow discharge light emission, negative glow is enhanced. Therefore, the light from the negative glow is used to display images or characters.
It should be noted that it is difficult to make a large liquid crystal display panel because of high rejection rate in the manufacturing process.
Further, in order to achieve a bright screen and high contrasted displayed images, it is necessary to use a back-light device.
A PDP uses a glow discharge device as its light source. Since the service life of a glow discharge device is short, the service life of a PDP is short. Further, the display screen of a PDP is dark because light from a negative glow is dark, in comparison with a CRT display.